1. Field of the Invention
This invention relates to a process for preparing a traction drive fluid which is used in power transmission systems.
2. Description of the Prior Art
As is well known in the art, automobiles have a number of power transmission and speed change gear systems such as transmission, shock absorbers, hydraulic stearing gears, clutches and other fluid actuators. For the transmission of power and speed change through these systems, it is usual to utilize point or line contact traction drive systems. A fluid for traction drive is incessantly supplied to contacting portions of the systems. This fluid serves as a lubricant when no load is applied and permits a friction and heat, which generate under a pressure of 15,000 to 30,000 kg/cm.sup.2 G on loading, to be reduced or dissipated. As fluids for traction drive, there have been proposed many compounds including various hydrocarbons and oxygen-containing hydrocarbons.
For example, there are known decalin, perhydroanthracene (U.S. Pat. No. 3,411,369), polycyclohexyls (ASLE Transition, 13, 105 (1970), U.S. Pat. No. 3,925,217), bicyclohexyl and dicyclohexylmethane (U.S. Pat. No. 3,440,894), 2,3-dicyclohexylbutane (Japanese Patent Application Laid-Open Gazette No. 46-4510), hydrogenated isobutylene oligomers (Japanese Patent Application Laid-Open Gazette Nos. 46-4766, 47-2164, 47-35661 and 47-2229), hydrogenated alpha-methylstyrene cyclized dimer (Japanese Patent Application Laid-Open Gazette No. 47-2229 and Japanese Patent Publication Gazette No. 47-35763), adamantanes (Japanese Patent Publication Gazette Nos. 48-42067, 48-42068 and 47-35763), esters having a cyclohexyl group (Japanese Patent Application Laid-Open Gazette No. 59-191797), and the like. These prior art traction drive fluids involve various problems such as a difficulty in obtaining inexpensive starting materials for the fluids in large amounts, a difficulty in operation of processes for the preparation of the fluids, a low traction coefficient (rolling friction coefficient) thereof, and the like.
Monsanto's and Sun Oil's Patents and Patent Applications (Japanese Patent Application Laid-Open Gazette No. 47-7664 and Japanese Patent Publication Gazette Nos. 46-339 and 47-35763) describe that polycyclic naphthene compounds are effective as a fluid for traction drive. In fact, a linear alpha-methylstyrene dimer or its analogues each in a nuclearly hydrogenated form are commercially available. However, these hydrogenated compounds leave a problem to be solved from economical and technical standpoints, i.e. it is economically difficult to obtain alpha-methylstyrene, and side reactions are involved in the process of dimerization of alpha-methylstyrene.
There have been studied for use as a fluid for traction drive not only nuclearly hydrogenated products of alpha-methylbenzylalkylbenzenes obtained by alkylating alkylbenzenes such as xylene, toluene and ethylbenzene, which are relatively readily available industrially, with styrene but also nuclearly hydrogenated products of compounds having three aromatic rings. However, they are not necessarily satisfactory in physical properties required for a fluid for traction drive and including, for example, a traction coefficient (rolling friction coefficient), viscosity, pour point, oxidation stability and thermal stability (Japanese Patent Application Laid-Open Gazette Nos. 55-43108 and 55-40726).